This invention relates to wood treatment compositions for imparting fire retardance to wood treated therewith. More particularly, the invention concerns fire-retardant wood treatment compositions which are partially reacted resin solutions based on melamine, formaldehyde and an oxyacid of phosphorous, i.e., phosphoric acid.
In recent years, a number of fire-retardant wood treatment compositions have been proposed based on amine and phosphorous compounds, and many of them amine-aldehyde-phosphorous condensation compounds. A common practice is to impregnate the wood with a solution of an incompletely reacted amine-aldehyde composition together with an oxyacid of phosphorous. After impregnation, the wood is dried and cured.
Illustrative of various prior art approaches are the treatment solutions disclosed in Goldstein et al, U.S. Pat. No. 2,917,408 prepared from a combination of dicyandiamide and phosphoric acid, and Goldstein et al, U.S. Pat. No. 3,159,503 prepared from a combination of dicyandiamide, phosphoric acid, and formaldehyde. In Juneja, U.S. Pat. No. 3,887,511, fire-retardant formulations are disclosed comprising a partially reacted mixture of urea, dicyandiamide, formaldehyde and phosphoric acid.
The prior art has recognized that in order to obtain better leach resistance, formulations may be prepared using melamine instead of urea. Thus, Juneja, in U.S. Pat. No. 3,832,316, discloses a fire-retardant composition comprising a partially reacted mixture of dicyandiamide, formaldehyde, melamine and phosphoric acid. A typical treatment formulation is prepared according to the disclosure of this patent by methylolating melamine and dicyandiamide by slowly mixing such with a formaldehyde solution adjusted to an alkaline pH. By performing the methylolation in an alkaline medium, condensation of the intredients is inhibited. Alkalinity in the formaldehyde solution is produced by the introduction of sodium hydroxide. After methylolation, the reaction mixture is cooled and phosphoric acid introduced slowly to produce the final intermediate stage resin solution. A treatment solution so produced has only limited storage life and commonly must be refrigerated to extend its storage life.
In Oberley, U.S. Pat. No. 4,010,296, a treatment composition is disclosed wherein a partially reacted mixture of monomethylol dicyandiamide, formaldehyde, and melamine is first prepared. This solution is then cooled to prepare it for shipment. Phosphoric acid may then be added to produce a final composition for wood treatment. While the mixture without phosphoric acid may have a limited degree of stability, as soon as phosphoric acid is added, storage life is sharply reduced unless refrigeration is employed.
Generally speaking, where melamine is utilized instead of urea, and because of the greater reactivity of melamine, storage life becomes a problem. To decrease the concentration of melamine in the composition so as to enhance storage life tends to decrease the leach resistance of the final composition when applied and cured on treated products. Further, to produce alkalinity in a solution by the addition of a material such as sodium hydroxide has disadvantages, since this introduces sodium ions into the final resin and thus solubility sites in the final resin.
Bottoming the instant invention is the appreciation that dicyandiamide may be advantageously employed in fire-retardant wood treatment solutions if such is converted to a derivative form exhibiting the characteristics of a strong base. In such derivative form, and upon the introduction of phosphoric acid to the treatment solution, a phosphate salt of the dicyandiamide derivative is produced effective to retard the mobility of phosphate ions in the solution. As a consequence, far greater storage life is obtained in the final treatment solution, even in a solution containing an appreciable amount of melamine. The solutions contemplated, nevertheless, are curable in a conventional manner, after being applied to a wooden article, to produce a treated product having excellent leach resistance, fire retardance, minimal hydroscopicity, and excellent resistance to blooming.
More specifically, and describing the preparation of compositions as contemplated, a solution may be prepared of dicyandiamide and water, with the initial pH of the solution adjusted to below 4, and preferably 3. On such being heated, and over a period of time, the dicyandiamide substantially quantitatively reacts with water to produce guanyl urea having the following structural formula: ##STR1## Guanyl urea, as well as methylolated guanyl urea, exhibits the characteristics of a strong base, and forms a phosphate salt on the addition of phosphoric acid to a solution containing the compound. As a consequence, treatment solutions comprising partially reacted product of methylolated guanyl urea, methylolated melamine and phosphoric acid exhibit prolonged storage lives, for example, up to six months under normal temperature conditions. Nevertheless, they are readily cured after application to a wood product to produce an insoluble resin with excellent fire-retardant characteristics.
Thus, a general object of the invention is to provide improved fire-retardant wood treatment solutions or compositions which are incompletely reacted amine-aldehyde compositions including an oxyacid of phosphorous.
It is a further object of the invention to provide a treatment composition for wood, to impart fire resistance thereto based on a derivative of dicyandiamide, melamine, formaldehyde and phosphoric acid, exhibiting an enhanced storage life.
A further object is to provide a wood treatment composition for imparting fire retardance to treated wood prepared by converting dicyandiamide in the presence of water and at an acid pH into guanyl urea, methylolating the guanyl urea and incorporating melamine with the methylolated guanyl urea, and thence incorporating with the mixture produced, phosphoric acid, to produce a partial reaction product suitable for the treatment of wood.
These and other objects and advantages are attained by the invention, which will become more fully apparent from the following description, taken in conjunction with specific examples included for the purpose of illustration only.
Explaining the present invention in greater detail, guanyl urea may be conveniently prepared from dicyandiamide by dissolving dicyandiamide in water, adjusting the pH of the resultant mixture to below about 4, and preferably within the range of 2 to 3, and heating the resultant mixture at a temperature above about 85.degree. C., and preferably at a temperature ranging from 90.degree. to 100.degree. C., for a period of approximately 30 minutes. One mole of dicyandiamide reacts with 1 mole of water to produce guanyl urea, and the conversion of the dicyandiamide to guanyl urea under these conditions is quantitative and irreversible. At the conclusion of the reaction, the pH of the result solution is only slightly on the acidic side, i.e., within the range of 5.5 to 6.5.
Because of solubility problems, the initial mixture of dicyandiamide and water which is prepared preferably has a concentration of dicyandiamide, on a weight basis, of about 50% or less. Although the conversion of dicyandiamide to guanyl urea will occur with relatively dilute solutions of dicyandiamide, normally solutions of less than about 30% dicyandiamide are not utilized, since this needlessly increases the volume of the mixture being handled, requiring the use of larger reactor vessels, introducing heating and cooling problems, etc.
The pH of the initial reaction mixture may be adjusted to the level indicated by the addition of phosphoric acid, or other conventional acids such as hydrochloric and sulfuric acids. Only a relatively small amount of acid is required to produce the pH adjustment. Thus, for example, for a reaction mixture containing 500 lbs. of dicyandiamide, typically only about 10 to 15 lbs. of phosphoric acid are required.
Guanyl urea may be methylolated to obtain essentially monomethylol guanyl urea by introducing formaldehyde to the guanyl urea solution prepared as above indicated. In the acid medium employed, monomethylol guanyl urea is essentially the only methylolated product obtained. Ordinarily, the usual 37% aqueous solution of formaldehyde may be used. Methylolation of a typical guanyl urea formaldehyde mixture to produce the guanyl urea methylolated product will occur over a period of about an hour, with the temperature of the mixture maintained within the range of about 70.degree.-80.degree. C.
After methylolation of the guanyl urea, melamine is introduced to the reaction mixture. The mole ratio of guanyl urea to melamine employed as reactants in making the treatment solution is within the range of 5:1 to 10:1. The mixture is cooled preferably before adding the melamine, to a temperature within the range of 50.degree.-60.degree. C. The mixture is maintained preferably at this temperature until the melamine has dissolved, and the mixture then preferably is cooled to about 30.degree. C. preparatory to the introduction of phosphoric acid.
The amount of formaldehyde introduced as a reactant to the reaction mixture produces a mole ratio of formaldehyde to combine to guanyl urea and melamine within the range of from 1:1 to 2:1. With methylolation of the guanyl urea using an amount of formaldehyde falling within a lower point of the indicated range, the amount of free formaldenyde remaining in the methylolation mixture prior to the introduction of the melamine is not substantial, but is sufficient to convert the melamine when such is added to a monomethylolated product, whereby it dissolves. With an increase in the mole ratio of formaldehyde to combine guanyl urea and melamine, more reactive mixtures are produced. With higher mole ratios, a greater amount of free formaldehyde remains in the reaction mixture prior to the introduction of the melamine. Optimum results are obtained, in terms of obtaining final solutions with good storage life and suitable reactivity, with mole ratios ranging from about 1.2 to 1.7:1.
A methylolated mixture of guanyl urea and melamine may alternatively be prepared by initially methylolating the guanyl urea utilizing a portion of the total formaldehyde to be employed, and using the remainder of the formaldehyde in the methylolating of the melamine in a separate vessel, with the methylolated products then combined. Ordinarily, no advantages are obtained by this procedure, however, Methylolation of the guanyl urea is performed before addition of the melamine by reason of the fact that melamine is the more reactive of the two materials, and it is desirable to obtain substantial monomethylolation of the guanyl urea before introduction of the melamine.
After preparation of the mixture of methylolated guanyl urea is produced by adding phosphoric acid to the mixture. The amount of phosphoric acid employed is within the range of from 1 to 1.5, preferably 1 to 1.4 moles of phosphoric acid per mole of amine compound in the reaction mixture, i.e., per combined moles of guanyl urea and melamine utilized in preparing the methylolated guanyl urea and melamine mixture. With the addition of phosphoric acid, and because of the characteristic of the strong base which methylolated guanyl urea possesses, a substantially quantitative reaction occurs with the production of a methylolated guanyl urea phosphate salt.
Mixtures resulting may be diluted with additional water at this time or at a later time to produce solutions used in the treatment of wood for fireproofing purposes. Commonly, wood treatment solutions are prepared having active solid concentrations ranging from about 10 to 30%. The term "active solid" refers to the nonvolatilized remainder of a sample dried at 105.degree. C. or 3 hours.